Filter presses are well known in the art and are used, for example, for dewatering effluents and sludges from industrial and municipal waste treatment processes, or for "polishing" process fluids. A typical filter press includes a horizontal row or stack of vertical filter plates. Opposite side faces of each plate are covered with filter cloth and the faces have recesses inwardly of the cloths, or are provided with frames to define recesses. During filtration, the plates are tightly compressed together and the recesses co-operate to define filter chambers between the plates. A liquid to be filtered is fed into the chambers and passes through the filter cloths. Drainage surfaces on both faces of the plate inwardly of the filter cloths allow the filtrate to pass to draining ports which communicate with filtrate outlets from the press.
Solids in the liquid feed stream accumulate on the outer faces of the filter cloths as so-called "cakes". At the end of a filtration portion of the press cycle, the feed is stopped, the plates are separated and the cakes of accumulated solids are dislodged from the filter cloths, and fall down between the plates.
Typically, the filter plates are suspended either from overhead beams, or from side bars or rails that extend longitudinally on opposite sides of the stack of filter plates. The rails extend between fixed heads which are at opposite ends of the stack of filter plates and sufficiently spaced therefrom to allow the plates to be separated for cake removal. A hydraulic ram extends between a fixed head at one end of the press and a movable head at the adjacent end of the stack of filter plates so that the plates can be compressed between the other fixed head and the movable head by extending the hydraulic ram. It will be understood that filtration is not in fact achieved by the pressure of the hydraulic ram but by the filter cloths. The ram merely holds the stack of filter plates together during filtration and then releases the plates for cake removal.
Interesting applications for filter presses exist in the mining industry for removal of base metal solids from aqueous streams. These streams may have relatively high solids concentrations (e.g. 60-70%) which means that the filter press cycle time is short. As such, it is advantageous to reduce the time required for filter cake removal and automate press operation as far as is possible.